TLR Activation Alters Bone Marrow-Derived Macrophage Differentiation.

Early exposure to inflammatory signals may have a lasting impact on immune function. Present throughout embryogenesis, macrophages are key cells providing innate immune protection to the developing fetus and newborn. Here, we have used an established model of macrophage development to test how early inflammatory signals can impact cellular differentiation and function. Bone marrow-derived macrophages were treated with Escherichia coli lipopolysaccharide (LPS) 2 days after initial isolation and culture. LPS treatment during this early stage of differentiation decreased the expression of CSF1R and increased that of the mature macrophage marker F4/80. These early changes in macrophage differentiation were also measured in cells from mice lacking IKKß, but the change in CSF1R expression after LPS treatment was blocked with MAPK inhibition. LPS-induced changes in macrophage marker expression persisted following LPS removal, suggesting that early inflammatory activation could induce a lasting developmental impact. Early LPS exposure inhibited macrophage phagocytosis of labeled E. coli while LPS had no effect on fully differentiated macrophages. Our data demonstrate that early inflammatory exposure to a microbial stimulus induce lasting phenotypic changes in macrophages.

Differential Immune Activation in Fetal Macrophage Populations.

Distinct macrophage subsets populate the developing embryo and fetus in distinct waves. However little is known about the functional differences between in utero macrophage populations or how they might contribute to fetal and neonatal immunity. Here we tested the innate immune response of mouse macrophages derived from the embryonic yolk sac and from fetal liver. When isolated from liver or lung, CD11bHI fetal liver derived macrophages responded to the TLR4 agonist LPS by expressing and releasing inflammatory cytokines. However F4/80HI macrophages from the yolk sac did not respond to LPS treatment. While differences in TLR4 expression did not appear to explain these data, F4/80HI macrophages had much lower NLRP3 inflammasome expression compared to CD11bHI macrophages. Gene expression profiling also demonstrated LPS-induced expression of inflammatory genes in CD11bHI macrophages, but not in F4/80HI cells. Genes expressed in LPS-treated CD11bHI macrophages were more likely to contain predicted NF-κB binding sites in their promoter regions. Our data show that CD11bHI macrophages derived from fetal liver are the major pro-inflammatory cells in the developing fetus. These findings could have important implications in better understanding the fetal inflammatory response and the unique features of neonatal immunity.